This invention relates generally to a cable seal, and more particularly to a cable seal for a pressure vessel assembly for housing electronic components in an underwater environment.
Underwater communication systems provide signal transmission between, or to, land-based positions. A typical system generally comprises a cable for signal transmission and one or more housings containing electrical components which are spaced along the cable between the land-based positions. The signal cable has a core which may include electrical conductors, fiber optic cable, or other signal transmission elements surrounded by a protective jacket. Metal strength members are disposed about the core of the cable between the cable core and the outer surface of the jacket. The strength members bear the tensile and compressive loads placed on the cable while in operation. The cable may carry various signals, including low voltage signals such as information and data signals, higher voltage signals for providing electrical power, or other types of signals.
The housings for the electronic components are referred to as pressure vessels. A pressure vessel is typically a cylindrical tube with open ends capped by circular bulkheads. The signal cable is terminated adjacent to each of the bulkheads in a termination assembly. The termination assemblies are secured to the bulkheads and provide mechanical continuity between the cable ends and the pressure vessel while relieving the stress on the signal transmission elements of the cable. The signal transmission elements pass into the pressure vessel through seals in the bulkheads for connection to the electronic components in the pressure vessel.
The pressure vessel assembly must protect the electronic components, signal cable and their connections from exposure to water at depths of up to 20,000 feet and pressures of up to 10,000 pounds per square inch for periods of up to 25 years. This harsh environment contributes to problems related to the performance reliability and product life of pressure vessel assemblies.
Conventional solutions to the problems of designing reliable and durable pressure vessel assemblies are plagued by high cost. The present practice is to use pressure vessels formed from beryllium-copper or titanium with polyethylene or gland cable seals and polyethylene-overmolded cable termination assemblies. Beryllium-copper or titanium is used for the pressure vessel because of their excellent resistance to corrosion in underwater applications. However, this material is very expensive, and machining is difficult. The polyehtylene or gland cable seals are expensive, consist of numerous parts, and are difficult and time-consuming to install. In addition to the seals, a water block of some sort must be used to prevent water ingress in the event of a cable cut.
The termination assemblies are overmolded with polyethylene to prevent water from accessing the internal portions of the signal cable and pressure vessel. In the overmolding process, high density polyethylene is molded around the cable termination assembly thereby sealing the areas between the outer surface of the cable jacket and the termination assembly. In some cases, portions of the cable and the pressure vessel are also overmolded with polyethylene. However, polyethylene overmolding is not cost effective in most applications because the required molding equipment is expensive and the process time consuming thereby restricting production rates. Moreover, the overmolding is a difficult process, requiring a high operator skill level and has yield and reliability problems.
For the foregoing reasons, there is a need for a reliable, long life, low cost pressure vessel assembly for housing electrical components in underwater communication systems. The new pressure vessel assembly must be capable of withstanding deep underwater pressures for many years. Seals for the passage of the signal cable transmission elements into the pressure vessel should be easy to install and effectively prevent moisture penetration. The termination assembly sealing process should be fast and simple to perform, requiring minimal operator skill level. Moreover, the components for sealing the cable termination assembly should be adaptable to seal various cable termination assembly types.